Method of manufacturing chip PTC thermistor

ABSTRACT

A method of manufacturing a chip PTC thermistor utilizes a piece of conductive polymer having a PTC characteristic and metal foils formed by patterning and provided on the upper and the lower sides of the piece under pressure integrally into a sheet, making a hole part in the sheet, applying a protective coating serving as plating resist to upper and lower sides of the sheet, forming an electrode on the sheet by electroplating, and cutting the sheet into a chip. The protective coating is made of a material applicable at a temperature equal to or lower than the melting point of the conductive polymer. The processing temperatures at the step from the step of making a hole part in the sheet to the pre-processing step of the step of forming an electrode by electroplating on the sheet are not above the melting point of the conductive polymer.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a chippositive temperature coefficient (hereinafter referred to as “PTC”)thermistor using electrically conductive polymer having a PTCcharacteristic.

BACKGROUND OF THE INVENTION

A PTC thermister composed of electrically conductive polymer is used asan overcurrent protective element in a variety of electronic devices. Anoperating principle is such that the electrically conductive polymerhaving a PTC characteristic heats up by itself when an excessive currentflows in an electric circuit, changing a resistance of its own into ahigh value due to a thermal expansion of the electrically conductivepolymer, thereby attenuating the current into a safe minute region.

A PTC thermistor of the prior art will be described hereinafter.

Japanese Patent Laid-open Publication, No. H09-503097 discloses anexample of a chip PTC thermistor of the prior art. It is a chip PTCthermister comprising a PTC element having a through-hole penetratingbetween a first surface and a second surface, and first and secondconductive members in a layer form, positioned inside of thethrough-hole, and connected physically as well as electrically to thefirst surface and the second surface of the PTC element.

FIG. 15(a) is a sectional view illustrating a chip PTC thermistor of theprior art, and FIG. 15(b) is a plan view of the same. In FIG. 15, areference numeral 81 represents an electrically conductive polymerhaving a PCT characteristic, reference numerals 82 a, 82 b, 82 c, and 82d represent electrodes composed of metallic foil, and reference numerals83 a and 83 b represent through-holes. Reference numerals 84 a and 84 bare conductive members formed by plating on insides of the through-holesand over the electrodes 82 a, 82 b, 82 c, and 82 d.

A method of manufacturing the abovedescribed chip PTC thermistor of theprior art will be described with reference to FIGS. 16(a) through 16(d)and FIGS. 17(a) through 17(c) which are procedural drawings showing amethod of manufacturing the chip PTC thermistor of the prior art.

First, polyethylene and carbon as electrically conductive particles areblended, and a sheet 91 shown in FIG. 16(a) is formed. Next, the sheet91 is sandwiched with two metallic foils 92, as shown in FIGS. 16(b) and16(c), and an integrated sheet 93 is formed by thermal-compressionmolding.

Next, through-holes 94 are perforated in a regularly arranged pattern onthe integrated sheet 93, as shown in FIG. 16(d), after it is irradiatedwith an electron beam. A plated film 95 is then formed on the insides ofthe through-holes 94 and on the metallic foils 92, as shown in FIG.17(a).

Etched grooves 96 are formed next in the metallic foils 92, as shown inFIG. 17(b).

The laminated product is now cut into individual pieces along cuttinglines 97 of a longitudinal direction and cutting lines 98 of a lateraldirection as shown in FIG. 17(b), to complete manufacturing of a chipPTC thermistor 99 of the prior art as shown in FIG. 17(c).

However, there has been a problem as described hereinafter with theconventional method of manufacturing the chip PTC thermistor, when aprotective coating is formed on the plated film 95 for the purpose ofpreventing a short circuit and the like.

That is, formation of the protective coating needs to be carried outonly after a pattern is formed by etching the metallic foil 92.Therefore, the protective coating is formed by screen-printing andthermally curing an epoxy base resin, after etched grooves are formed inthe metallic foil 92. The problem occurs in this process that a crackmay develop in the plated film 95 formed in the through holes 94 due toa mechanical stress generated by thermal expansion because of the heatapplied when thermally curing the sheet 91.

It is conceivable to use a method wherein the etched grooves 96 areformed in the metallic foil, the protective coating is formed next, andthe plated film 95 is formed thereafter, in order to prevent the crackfrom developing in the plated film 95. However, a problem has yetremained unresolved that the plated film 95 can not be formed uniformlyon inner surfaces of the through-holes 94 in this method. It is presumedthat this is because a surface of the sheet 91 loses an electricconductivity, as a result of the heat during the thermal setting of theprotective coating, which causes polyethylene element in the sheet 91 tomigrate toward the surface of the sheet 91 exposed on the inner surfacesof the through-holes 94.

An object of the present invention is to solve the foregoing problem ofthe prior art method, and to provide a method of manufacturing a chipPTC thermistor having superior reliability of connection, as it does notcause a crack in the electrode connecting between upper and lowerelectrodes when the protective coating is formed on the metallic foil,and it is capable of uniformly forming a film by electrolytic platingeven on a portion of the electrically conductive polymer on an innersurface of the opening when the electrode is formed.

SUMMARY OF THE INVENTION

A method of the present invention for manufacturing a chip PTCthermistor comprises includes the steps of;

forming a sheet by sandwiching an upper surface and a lower surface ofan electrically conductive polymer having a PTC characteristic withmetallic foils, on which a pattern is formed in advance, and integratingthem by thermal-compression molding;

providing an opening in the integrated sheet;

forming a protective coating, also serving as plating resist, on anupper and lower surfaces of the sheet in which the opening is provided;

forming an electrode by electrolytic plating on the sheet on which theprotective coating serving also as plating resist is formed; and

cutting the sheet, on which the electrode is formed, into individualpieces.

In addition, a material that is capable of being formed at a temperaturebelow a melting point of the electrically conductive polymer is used fora material of the protective coating, also serving as plating resist, inthe step of forming the protective coating also serving as platingresist. Furthermore, a processing temperature is maintained in such amanner as not to exceed the melting point of the electrically conductivepolymer in each step of the preparatory processes from the step ofproviding the opening in the integrated sheet, up to the step of formingthe electrode by electrolytic plating on the sheet, on which theprotective coating serving also as plating resist, is formed. Themanufacturing method of the present invention provides the chip PTCthermistor having a superior reliability of connection, since it doesnot cause a crack in the electrode formed by electrolytic plating, andis capable of uniformly forming a film of the electrolytic plating evenon the portion of the electrically conductive polymer on the insidesurface of the opening when the electrode is formed. In addition, thepresent invention can eliminate waste liquid that is otherwise producedif wet patterning is used for the metallic foil in the process ofmanufacturing the chip PTC thermistor, since the present method uses themetallic foil patterned in advance by die-cutting to manufacture theintegrated sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a perspective view of a chip PTC thermistor in a firstexemplary embodiment of the present invention;

FIG. 1(b) is a sectional view taken along a line A-A′ in FIG. 1(a);

FIGS. 2(a) through 2(d) are procedural drawings showing a method ofmanufacturing the chip PTC thermistor in the first exemplary embodimentof the present invention;

FIGS. 3(a) through 3(d) are perspective views showing the method ofmanufacturing the chip PTC thermistor in the first exemplary embodimentof the present invention;

FIG. 4 is a perspective view of a chip PTC illustrating an example of adefectively formed electrode;

FIGS. 5(a) through 5(e), are perspective views showing a method ofmanufacturing a chip PTC thermistor in a second exemplary embodiment ofthe present invention;

FIGS. 6(a) through 6(d) are perspective views showing the method ofmanufacturing the chip PTC thermistor in the second exemplary embodimentof the present invention;

FIG. 7(a) is a perspective view of a chip PTC thermistor in a thirdexemplary embodiment of the present invention;

FIG. 7(b) is a sectional view taken along a line B-B′ in FIG. 7(a);

FIGS. 8(a) through 8(d) are perspective views showing a method ofmanufacturing a chip PTC thermister in the third exemplary embodiment ofthe present invention;

FIGS. 9(a) through 9(d) are perspective views showing the method ofmanufacturing the chip PTC thermistor in the third exemplary embodimentof the present invention;

FIGS. 10(a) through 10(e) are perspective views showing a method ofmanufacturing a chip PTC thermistor in a fourth exemplary embodiment ofthe present invention;

FIGS. 11(a) through 11(d) are perspective views showing the method ofmanufacturing the chip PTC thermistor in the fourth exemplary embodimentof the present invention;

FIGS. 12(a) through 12(d) are perspective views showing a method ofmanufacturing a chip PTC thermistor in a fifth exemplary embodiment ofthe present invention;

FIGS. 13(a) through 13(d) are also perspective views showing the methodof manufacturing the chip PTC thermistor in the fifth exemplaryembodiment of the present invention;

FIG. 14(a) is a graph showing a thickness of electrode in the case aplating resist for masking is provided;

FIG. 14(b) is another graph showing a thickness of electrode whenmanufactured without providing a plating resist for masking;

FIG. 15(a) is a sectional view of a chip PTC thermistor of the priorart;

FIG. 15(b) is a plan view of the chip PTC thermistor of the prior art;

FIGS. 16(a) through 16(d) are perspective views showing a method ofmanufacturing the chip PTC thermistor of the prior art; and

FIGS. 17(a) through 17(c) are perspective views showing a method ofmanufacturing a chip PTC thermistor of the prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIRST EXEMPLARY EMBODIMENT

A chip PTC thermistor and a method of manufacturing the same in a firstexemplary embodiment of this invention will be described hereinafter, byreferring to the accompanying figures.

FIG. 1(a) is a perspective view of the chip PTC thermistor, and FIG.1(b) is a sectional view taken along a line A-A′ in FIG. 1(a), in thefirst exemplary embodiment of this invention.

In FIGS. 1(a) and 1(b), a reference numeral 11 represents anelectrically conductive polymer (melting point: approx. 135° C.) in acuboidal shape having a PTC characteristic, comprising a compound ofhigh density polyethylene (melting point: approx. 135° C.), i.e.crystalline polymer, and carbon black, i.e. electrically conductiveparticles. A reference numeral 12 a represents a first main electrodelocated on a first surface of the electrically conductive polymer 11. Areference numeral 12 b represents a first sub-electrode located on thesame surface as the first main electrode 12 a, but independently fromthe first main electrode 12 a. A reference numeral 12 c is a second mainelectrode located on a second surface opposite to the first surface ofthe electrically conductive polymer 11, and a reference numeral 12 d isa second sub-electrode located on the same surface as the second mainelectrode 12 c, but independently from the second main electrode 12 c.Each of the electrodes consists of a metallic foil such as electrolyticcopper foil.

A first side electrode 13 a consisting of a layer of electrolyticallyplated nickel, is disposed in such a manner as to surround over anentire side surface of the electrically conductive polymer 11, an edgeportion of the first main electrode 12 a and the second sub-electrode 12d, and to electrically connect the first main electrode 12 a and thesecond sub-electrode 12 d.

A second side electrode 13 b consisting of a layer of electrolyticallyplated nickel, is also disposed in such a manner as to surround over anentire surface of another side opposite to the first side electrode 13 aof the electrically conductive polymer 11, an edge portion of the secondmain electrode 12 c and the first sub-electrode 12 b, and toelectrically connect the second main electrode 12 c and the firstsub-electrode 12 b.

Reference numerals 14 a and 14 b are a first protective coating and asecond protective coating respectively in green color, both serving alsoas plating resist, composed of polyester-based resin provided onoutermost layers of the first surface and the second surface of theelectrically conductive polymer 11. Incidentally, the first sideelectrode 13 a and the second side electrode 13 b may take any formsthat can be provided on portions of the side surfaces of the PTCthermistor, or on the inside of the through-holes of the prior artstructure.

A method of the first exemplary embodiment of this invention formanufacturing the chip PTC thermistor will now be described by referringto the accompanying figures.

FIGS. 2(a) through 2(d) and FIGS. 3(a) through 3(d) are showing themethod of manufacturing the chip PTC thermistor in the first exemplaryembodiment of this invention.

First, a 42 weight % of high density polyethylene (melting point:approx. 135° C.) having a crystallinity of 70 to 90%, a 57 weight %carbon black having a mean particle diameter of 58 nm and a specificsurface area of 38 m²/g, manufactured by furnace method, and a 1 weight% of antioxidant were kneaded for about 20 minutes with two-roll millheated to approximately 170° C. The kneaded substance in a sheet-formwas taken out from the two-roll mill, and an electrically conductivepolymer 21 (melting point: approx. 135° C.) shown in FIG. 2(a), in asheet-form having a thickness of approximately 0.16 mm was produced.

Next, a metallic foil 22 shown in FIG. 2(b) was made from electrolyticcopper foil of approx. 80 μm by pattern forming with a press forming.

Next, a sheet 23 integrated as shown in FIG. 2(d) was produced byoverlaying one each of the metallic foil 22 on top and bottom of thesheet-formed electrically conductive polymer 21, as shown in FIG. 2(c),and subjecting them to a compression molding for approx. 1 minute undera condition of 140° C. to 150° C. in temperature, approx. 20 torr indegree of vacuum, and approx. 50 kg/cm² in surface pressure.

After the integrated sheet 23 was thermally treated (approx. 20 minutesat 100° C. to 115° C.), it was irradiated with approx. 40 Mrad ofelectron beam in an electron beam irradiating apparatus to completecress-linking of the high density polyethylene.

Long slit openings 24 were then formed at regular intervals in theintegrated sheet 23, as shown in FIG. 3(a), by using a dicing machine, amilling machine, or the like, while cooling it with water. In theprocess of forming the openings 24, certain desired portions were leftuncut in a longitudinal direction of the openings 24. In a process ofrinsing and drying after the openings 24 were cut, the work was carriedout at such a temperature that a temperature of the electricallyconductive polymer 21 does not rise beyond the melting point (135° C.)of the electrically conductive polymer 21.

Next, an upper surface and a lower surface of the sheet 23 provided withthe openings 24 was screen-printed with green colored paste of polyesterbased thermo-setting resin, except for an area surrounding the opening24, as shown in FIG. 3(b), and a protective coating 25 also serving asplating resist was formed by curing it (at 125° C. to 130° C. forapprox. 10 minutes) in a curing oven.

Then, a side electrode 26 was formed, as shown in FIG. 3(c), on aportion of the integrated sheet 23, where the protective coating 25 alsoserving as plating resist is not formed, and on inner surfaces of theopenings 24. The side electrode 26 was formed by electrolytic nickelplating in a thickness of approx. 15 μm in a sulfamic acid nickel bathunder a condition of an electric current density of 4 A/dm², for about30 minutes

The sheet 23 of FIG. 3(c) was divided, thereafter, into individualpieces with a dicing machine to complete a chip PTC thermistor 27 shownin FIG. 3(d).

Hereinafter an advantage of the foregoing processes will be explained,wherein a temperature of the electrically conductive polymer 21 is somaintained as not to exceed the melting point (135° C. of theelectrically conductive polymer 21 during the preparatory processeswhich include the steps from the forming of the openings 24 shown inFIG. 3(a) to the forming of the side electrode 26 shown in FIG. 3(c), byadopting the protective coating, also serving as plating resist, capableof being formed at a temperature equal to or lower than the meltingpoint 135° C. of the electrically conductive polymer.

In comparison, a protective coating 25, also serving as plating resist,was formed by screen-printing a resin paste of the ordinary epoxy basedthermo-setting resin and curing it (at 140° C. to 150° C. for approx. 10minutes) in an oven, in the step of forming the protective coating 25,also serving as plating resist, shown in FIG. 3(b). The followingproblem arose in this case, in the step of forming the side electrode26.

First of all, FIG. 4 shows an example of defects developed when the sideelectrodes 13 a and 13 b of the chip PTC thermistor were formed.

In FIG. 4, a reference numeral 15 represents a defective portion formedin the side electrodes 13 a and 13 b. Although nickel plating isproperly formed on the main electrodes 12 a, 12 c, and thesub-electrodes 12 b and 12 d, the same nickel plating is formed onlypartially on the electrically conductive polymer 11. Therefore, the mainelectrodes 12 a and 12 c, and the sub-electrodes 12 b and 12 d have notconnected both electrically and physically. This is caused by the factthat the electrically conductive polymer 11 is unable to keep anelectrical conductivity in its surface, while the main electrodes 12 aand 12 c as well as the sub-electrodes 12 b and 12 d, being metal parts,keep high electrical conductivity. It is presumed that the surface ofthe electrically conductive polymer 11 is unable to maintain theelectrical conductivity, because the electrically conductive polymer 11is heated beyond the melting point of 135° C. under the processingtemperature of 140° C. to 150° C. for 10 minutes, which causes thepolyethylene element within the electrically conductive polymer 11 tomigrate toward its surface. Naturally, a film of the electrolyticplating is not formed on the portion where electrical conductivity islost, thereby giving rise to defective formation of the side electrodes13 a and 13 b.

There are two important points described below in order to avoid theforegoing problem, and to ensure reliability of connection bysuccessfully forming the side electrode 26.

The first one is to use a protective coating 25 serving as platingresist that can be formed at a temperature equal to or less than themelting point of 135° C. of the electrically conductive polymer 21.

The second one is to prevent temperature of the electrically conductivepolymer 21 being heated up to the melting point (135° C.) or higherduring the steps from the forming of the openings 24 through thecompleting the formation of the side electrode 26.

It is therefore necessary to prevent the temperature of the electricallyconductive polymer 21 from being heated to the melting point (135° C.)or higher even with a processing temperature in any step other than thestep of forming the protective coating 25, also serving as platingresist, such as a processing temperature when rinsing and drying it, andso on, after the dicing, for the same reason as described above.

Because of the above reason, the first exemplary embodiment of thisinvention does not cause a crack in the side electrode 26 composed of alayer of electrolytically plated nickel, even if the protective coating25, also serving as plating resist, is formed upon consideration of ashort circuiting due to deviation in a position of soldering on aprinted circuit board.

This exemplary embodiment can also provide the chip PTC thermistorhaving superior reliability of connection, as it does not cause such aproblem as not forming the side electrode 26 uniformly on the innersurface of the opening 24.

An advantage of the first exemplary embodiment of this invention forforming the side electrode 26 with the layer of electrolytically platednickel will be described hereinafter.

First of all, it requires approx. 30 minutes with an electric currentdensity of about 4.0 A/dm² in order to form the side electrode 26 in athickness of 15 μm in the case of electrolytic nickel plating in thestep of forming the side electrode. On the contrary, it requires morethan twice as long to approx. 80 minutes with an electric currentdensity of approx. 1.5 A/dm² in the case of electrolytic copper plating.Defects such as yellowing, abnormal deposition, and the like of theplating occur if the electric current density for the electrolyticcopper plating is increased to about 4.0 A/dm² for the purpose offorming the plated film within a short period of time. In the case ofthe electrolytic copper plating, therefore, it is difficult to form aplated film of the same thickness as the electrolytic nickel plating ina short period of time.

In addition, a thermal-shock test (between −40° C. for 30 minutes and+125° C. for 30 minutes) was performed on samples having a sideelectrode of the same. thickness prepared with a layer of theelectrolytically plated nickel, and a layer of the electrolyticallyplated copper. No defect such as a crack, etc. occurred on any of theelectrode samples formed with the layer of electrolytically platednickel, upon observation of polished sections after completion of a100-cycle and a 250-cycle thermal shock tests. In the case of thesamples formed with the layer of electrolytically plated copper,however, a crack occurred, as was observed on polished sections after acompletion of the 100-cycle thermal shock test. Moreover, it wasobserved that some of the samples show a complete disconnection due tocracks after the 250-cycle thermal shock test.

The above results suffice it to note that the side electrode 26 formedwith the layer of electrolytically plated nickel has such effects asshortening a manufacturing time and improving the reliability ofconnection.

SECOND EXEMPLARY EMBODIMENT

A method of manufacturing a chip PTC thermistor in a second exemplaryembodiment of the present invention will be described next by referringto FIGS. 5 and 6.

FIGS. 5(a) through 5(e) and FIGS. 6(a) through 6(d) show the method ofmanufacturing the chip PTC thermistor in the second exemplary embodimentof this invention.

An electrically conductive polymer 31 (melting point: approx. 135° C.)shown in FIG. 5(a), in a sheet-form having a thickness of approximately0.16 mm was produced in the same manner as in the first exemplaryembodiment.

Next, an integrated sheet 33 shown in FIG. 5(d) was produced byoverlaying a metallic foil 32 shown in FIG. 5(b) composed of anelectrolytic copper foil of approx. 80 μm on top and bottom of theelectrically conductive polymer 31, as shown in FIG. 5(c), andsubjecting them to a thermal-compression molding for approx. 1 minute at140° C. to 150° C. in temperature, approx. 40 torr in degree of vacuum,and approx. 50 kg/cm² in surface, pressure.

The metallic foils 32 on the top and the bottom surfaces of theintegrated sheet 33 were etched by the photolithographic process to forma pattern as shown in FIG. 5(e).

The sheet 33, formed with the pattern, was thermally treated (at 100° C.to 115° C. for approx. 20 minutes), and it was irradiated with approx.40 Mrad of electron beam in an electron beam irradiating apparatus tocomplete cress-linking of the high density polyethylene. A chip PTCthermistor 37 shown in FIG. 6(d) was obtained by taking manufacturingsteps thereafter, as shown in FIGS. 6(a) through 6(d), in the samemanner as the first exemplary embodiment of this invention.

The chip PTC thermistor 37 manufactured in the manner as described abovehas similar effects as those of the first exemplary embodiment of thisinvention. That is, this exemplary embodiment can provide for the chipPTC thermistor having superior reliability of connection, as it does notcause such a problem as having a crack in a side electrode 36 composedof a layer of electrolytically plated nickel, and a defect in formationoft he side electrode 36, even if a protective coating 35, also servingas plating resist, is formed upon consideration of a short circuitingdue to deviation in a position of soldering on a printed wiring board.

THIRD EXEMPLARY EMBODIMENT

A chip PTC thermistor and a method of manufacturing the same in a thirdexemplary embodiment of this invention will be described next byreferring to the accompanying figures. FIG. 7(a) is a perspective viewof the chip PTC thermistor, and FIG. 7(b) is a sectional view takenalong a line B-B′ in FIG. 7(a), in the third exemplary embodiment ofthis invention.

A structure of the chip PTC thermistor shown in FIG. 7(a) and 7(b) isthe same in principle with that of the first exemplary embodiment. Thisexemplary embodiment differs from the first exemplary embodiment, inthat first and second protective coatings 44 a and 44 b, also serving asplating resist of green color, provided on outermost layers of a firstsurface and a second surface of an electrically conductive polymer 41are composed of epoxy based resin.

The method of the third exemplary embodiment of this invention formanufacturing the chip PTC thermister will be described next byreferring to FIGS. 8(a) through 8(d) and FIGS. 9(a) through 9(d).

Manufacturing processes of this exemplary embodiment are the same asthose of the first exemplary embodiment, up to the step for irradiatingthe electron beam on an integrated sheet.

Next, an upper surface and a lower surface of an integrated compositesheet 53 was screen-printed with green colored paste of epoxy basedthermo-setting resin, and a protective coating 54, also serving asplating resist, was formed by curing it (at 145° C. to 150° C. forapprox. 10 minutes) in a curing oven, as shown in FIG. 9(a).

Long slit openings 55 were then formed at regular intervals in theintegrated sheet 53, as shown in FIG. 9(b), by using a dicing machine, amilling machine, or the like, while cooling it with water. In theprocess of forming the openings 55, predetermined portions were leftuncut in a longitudinal direction of the openings 55. In the case ofrinsing and drying it after the openings 55 were cut, the work wascarried out at such a temperature that a temperature of an electricallyconductive polymer 51 does not rise beyond the melting point (135° C.)of the electrically conductive polymer 51.

Then, a side electrode 56 comprising a layer of electrolytically platednickel in a thickness of approx. 15 μm was formed, as shown in FIG.9(c), on a portion of the sheet 53, where the protective coating 54,also serving as plating resist, is not formed, and on inner walls of theopenings 55 by nickel plating in a sulfamic acid nickel bath under acondition of an electric current density of 4 A/dm², for about 30minutes.

The sheet 53 of FIG. 9(c) was divided, thereafter, into individualpieces with a dicing machine to complete a chip PTC thermister 57 shownin FIG. 9(d).

An effect of the manufacturing method shown in this third exemplaryembodiment of the present invention will be described hereinafter.

First, there is a necessity, for the same reason as what has beendescribed in the first exemplary embodiment of this invention, thattemperature of the electrically conductive polymer 51 is maintained soas not to exceed the melting point (135° C.) of the electricallyconductive polymer 51 during the preparatory processes from the step offorming the openings 55 shown in FIG. 9(b) to the step of forming a sideelectrode 56 shown in FIG. 9(c). The purpose of this is to properly formthe side electrode 56 that is an essential point to assure reliabilityof connection.

Next, an advantage of forming the protective coating 54, also serving asplating resist, shown in FIG. 9(a), before cutting the openings 55 shownin FIG. 9(b), will be described.

It becomes unnecessary to restrict material used for forming theprotective coating 54, also serving as plating resist, to such amaterial that can be formed at a temperature below the melting point(135° C.) of the electrically conductive polymer 51, when the protectivecoating 54 serving as plating resist is formed before cutting theopenings 55. Therefore, this gives an advantage that material can beselected freely among a variety of general resin materials that can beformed at about 150° C., according to characteristics necessary withrespect to adhesiveness, mechanical strength, and so on. Furthermore, itcan give such an effect as to shorten a curing time and to improveadhesion by increasing a curing temperature to approx. 150° C. for amaterial that can be formed at the curing temperature of 130° C. orbelow.

FOURTH EXEMPLARY EMBODIMENT

A method of a fourth exemplary embodiment of this invention formanufacturing a chip PTC thermistor will be described next by referringto FIGS. 10(a) through 10(e) and FIGS. 11(a) through 11(d).Manufacturing processes of this exemplary embodiment are the same asthose of the second exemplary embodiment, up to the step for irradiatingelectron beam on an integrated sheet.

A chip PTC thermistor 67 shown in FIG. 11(d) was obtained by taking themanufacturing steps shown in FIG. 11(a) through 11(d) in the same manneras those of the third exemplary embodiment of this invention.

The chip PTC thermistor 67 manufactured in the manner as described abovehas similar effects as those of the third exemplary embodiment of thisinvention. That is, this exemplary embodiment can provide a chip PTCthermistor having superior reliability of connection, as it does notcause such a problem as having a crack in a side electrode 66 composedof a layer of electrolytically plated nickel, and a defective formationof the side electrode, in that the side electrode 36 can not be formeduniformly over an inner surface of openings 65, even if a protectivecoating, also serving as plating resist, is formed upon consideration ofa short circuiting, etc. due to deviation in a position of soldering ona printed wiring board.

In addition, it becomes unnecessary to restrict material used forforming the protective coating 64, also serving as plating resist, tosuch a material that can be formed at a temperature below the meltingpoint (135° C.) of an electrically conductive polymer 51, when theprotective coating 64 also serving as plating resist is formed beforecutting the openings 65. Therefore, this gives an advantage thatmaterial can be selected freely among a variety of general resinmaterials that can be formed at about 150° C., according tocharacteristics necessary with respect to adhesion, mechanical strength,and so on. Furthermore, it can give such an effect as to shorten acuring time and to improve adhesion by increasing a curing temperatureto approx. 150° C. for a material that can be formed at the curingtemperature of 130° C. or below.

FIFTH EXEMPLARY EMBODIMENT

A method of a fifth exemplary embodiment of this invention formanufacturing a chip PTC thermistor will be described next by referringto FIGS. 12(a) through 12(d) and FIGS. 13(a) through 13(d).Manufacturing processes of this exemplary embodiment are the same asthose of the first exemplary embodiment, up to the step for forming anopening 74.

Next, a protective coating 75, also serving as plating resist, andanother plating resist 76 for masking purposes were formed at the sametime with the same material by screen-printing green colored paste ofpolyester base thermo-setting resin on an upper surface and a lowersurface of a sheet 73 provided with the openings 74, and by curing it(at 125° C. to 130° C. for approx. 10 minutes) in a curing oven, asshown in FIG. 13(b).

During this process, the protective coating 75, also serving as platingresist, was formed on a product part except for an area surrounding theopenings 74, and the plating resist 76 for masking was formed on an areanot usable for the product part of the sheet 73 with a contact point 79for plating left intact.

Then, a side electrode 77 was formed, as shown in FIG. 13(c), on aportion of the sheet 73, where the protective coating 75, also servingas plating resist, and the plating resist 76 for masking are not formed,and on inner walls of the openings 74 by plating with nickel in athickness of approx. 15 μm. The nickel plating was made in a sulfamicacid nickel bath under a condition of an electric current density of 4A/dm², for about 30 minutes.

The sheet 73 of FIG. 13(c) was divided, thereafter, into individualpieces with a dicing machine to complete a chip PTC thermistor 78 shownin FIG. 13(d).

Described hereinafter is an effect of the plating resist 76 for masking.

Two kinds of samples were prepared for comparison purposes, wherein theside electrode 77 was formed after forming the plating resist 76 formasking on the area not usable for the product part of the sheet 73, forone case, and the side electrode 77 was formed without forming theplating resist 76 for masking in another case. Fifty (50) samples weretaken for each of the groups, and the thicknesses of the side electrodes77 were measured by observing their sections. The results are shown inFIGS. 14(a) and 14(b). As it is obvious from FIGS. 14(a) and 14(b), thecase where the plating resist 76 for masking was formed shows a smallerdeviation in thickness of the side electrode 77. The reason of this isthat a presence of the plating resist 76 for masking makes the electriccurrent density uniform around an area of the side electrode 77 duringthe plating process.

Accordingly, the fifth exemplary embodiment for this invention canprovide the chip PTC, thermistor exhibiting stable reliability ofconnection, since it can reduce the deviation in thickness of the sideelectrode 77, in addition to the effects provided by the first to thefourth exemplary embodiments.

The protective coating 75, also serving as plating resist, and theplating resist 76 for masking may be formed individually with differentmaterials. However, a positional relation can be established firmlybetween the protective coating 75, also serving as plating resist, andthe plating resist 76 for masking, if they are formed at the same timewith the same material as in the case of this fifth exemplary embodimentof the invention. This method can therefore make the thickness of theside electrode more uniform as compared to the case in which they areformed individually. Moreover, it also provides an effect of a costreduction by reducing the manufacturing steps, etc., since theprotective coating 75 and the plating resist 76 for masking can beformed with a single step of printing.

Besides, although the polyester base thermo-setting resin was used forthe protective coating 75 also serving as plating resist, and theplating resist 76 for masking, in the present exemplary embodiment, anyother kind of epoxy based resin may also be used, as it is superior inits properties of heat resistance, chemical resistance, and adhesion, asdescribed in the foregoing third and the fourth exemplary embodiments.

The manufacturing method does not cause a crack in the electrode due toan effect of heat during formation of the protective coating, servingalso as plating resist, since the electrode is formed by plating onlyafter the protective coating serving also as plating resist is formed.

Moreover, this method is able to form the electrode uniformly, since itmaintains an electrical conductivity on a surface of the electricallyconductive polymer, by way of controlling the processing temperature insuch a manner as to prevent polymer in the electrically conductivepolymer from migrating toward a surface of the electrically conductivepolymer exposed on an inner surface of the opening. As a result, aneffect capable of manufacturing the chip PTC thermistor having asuperior reliability of connection can be obtained.

INDUSTRIAL APPLICABILITY

As has been described, a method of the present invention formanufacturing a chip PTC thermistor provides an effect of providing amanufacturing method of the chip PTC thermistor having superiorreliability in connection, at low cost with excellent mass-productivity.Accordingly, the chip PTC thermistor can be used effectively as anover-current protective element in a variety of electronic devices.

What is claimed is:
 1. A method of manufacturing a chip PTC thermistor comprising, in sequence: forming a sheet by sandwiching an upper surface and a lower surface of an electrically conductive polymer having a PTC characteristic with metallic foils having a pattern formed thereon, and integrating said sheet by thermal-compression molding; providing at least one opening in said integrated sheet; forming a protective coating on respective first portions of an upper surface and a lower surface of said sheet provided with said opening, said protective coating also serving as a first plating resist, said protective coating formed at a temperature below a melting point of said electrically conductive polymer; forming an electrode by electrolytic plating on said sheet having said protective coating formed thereon; and dividing said sheet having said electrode formed thereon into individual pieces, wherein said providing an opening, forming a protective coating, and forming an electrode, all conducted at a processing temperature not exceeding the melting point of said electrically conductive polymer.
 2. The method of manufacturing a chip PTC thermister according to claim 1, wherein forming said electrode by electrolytic plating comprises electrolytic nickel plating.
 3. The method of manufacturing a chip PTC thermistor according to claim 2, further comprising after forming said integrated sheet and before forming said electrode by electrolytic plating, forming a plating resist as a mask on respective second portions of the upper and the lower surfaces of said sheet, said respective second portions being other than a part of the chip PTC thermistor.
 4. The method of manufacturing a chip PTC thermistor according to claim 3, wherein forming a second plating resist occurs at a time of forming a protective coating.
 5. The method of manufacturing a chip PTC thermister according to claim 2, wherein the step of forming said sheet forming a sheet under a pressure lower than an atmospheric pressure.
 6. The method of manufacturing a chip PTC thermister according to claim 5, wherein said pressure lower than the atmospheric pressure is at least 40 torr.
 7. The method of manufacturing a chip PTC thermistor according to claim 1, further comprising after forming said integrated sheet and before forming said electrode by electrolytic plating, forming a second plating resist as a mask on respective second portions of the upper and the lower surfaces of said sheet, said respective second portions being other than a part of the chip PTC thermistor.
 8. The method of manufacturing a chip PTC thermistor according to claim 7, wherein forming a second plating resist occurs at a time of forming a protective coating.
 9. The method of manufacturing a chip PTC thermister according to claim 1, wherein forming a sheet comprises forming a sheet under a reduced pressure.
 10. The method of manufacturing a chip PTC thermister according to claim 9, wherein said pressure is at least 40 torr.
 11. A method of manufacturing a chip PTC thermistor comprising, in sequence: forming a sheet by sandwiching an upper surface and a lower surface of an electrically conductive polymer having a PTC characteristic with metallic foils, and integrating them by thermal-compression molding; forming a pattern by etching said metallic foils on an upper surface and a lower surface of said integrated sheet; providing at least one opening in said sheet having the pattern formed thereon; forming a protective coating, also serving as a plating resist, on the upper and the lower surfaces of said sheet provided with said opening, said protective coating formed at a temperature below a melting point of said electrically conductive polymer; forming an electrode by electrolytic plating on said sheet having said protective coating, serving also as plating resist, formed thereon; and dividing said sheet having said electrode formed thereon into individual pieces, wherein said forming a pattern, providing an opening, forming a protective coating, and forming an electrode, all conducted at a processing temperature not exceeding the melting point of said electrically conductive polymer.
 12. A method of manufacturing a chip PTC thermister comprising in sequence: forming a sheet by sandwiching an upper surface and a lower surface of an electrically conductive polymer having a PTC characteristic with metallic foils having a pattern formed thereon, and integrating them by thermal-compression forming; forming a protective coating, also serving as plating resist, on an upper and a lower surface of said integrated sheet, said protective coating formed at a temperature below a melting point of said electrically conductive polymer; providing at least one opening in said sheet having said protective coating, serving also as plating resist, formed thereon; forming an electrode by electrolytic plating on said sheet provided with said opening; and dividing said sheet having said electrode formed thereon into individual pieces, wherein said forming a protective coating, providing an opening, and forming an electrode, all conducted at a processing temperature not exceeding the melting point of said electrically conductive polymer.
 13. A method of manufacturing a chip PTC thermister comprising in sequence: forming a sheet by sandwiching an upper surface and a lower surface of an electrically conductive polymer having a PTC characteristic with metallic foils, and integrating them by thermal-compression forming; forming a pattern by etching said metallic foils on an upper surface and a lower surface of said integrated sheet; forming a protective coating, also serving as plating resist, on the upper and the lower surfaces of said sheet having the pattern formed thereon, said protective coating formed at a temperature below a melting point of said electrically conductive polymer; providing at least one opening in said sheet having said protective coating, serving also as plating resist, formed thereon; forming an electrode by electrolytic plating on said sheet provided with said opening; and dividing said sheet having said electrode formed thereon into individual pieces, wherein said forming a protective coating, providing an opening, and forming an electrode, all conducted at a processing temperature not exceeding the melting point of said electrically conductive polymer. 